1. Field of the Invention
This invention relates to a buried stripe type semiconductor laser device and a method of producing a buried stripe type semiconductor laser device.
2. Description of the Prior Art
In recent years, semiconductor laser devices are widely used as light sources for various consumer electronics and industrial apparatuses in such applications as optical communications and information processing. In order to meet the device characteristics (such as high output power, high efficiency, and high-speed modulation) required of these semiconductor laser devices, it is necessary to stabilize a transverse oscillation mode and reduce a threshold current. In particular, stabilization of the transverse oscillation mode is important, because it has a close relation with fiber coupling efficiency in optical communications and with optical beam astigmatism in information processing.
Various structures of semiconductor laser devices have been proposed so far for controlling the transverse oscillation mode. Generally, semiconductor laser devices having the Fabry-Perot resonator structure are classified into several types, such as a gain waveguide type having a current confinement structure, a built-in refractive index waveguide type having transverse distribution in a refractive index perpendicular to the cavity length of the resonator, and a refractive index waveguide type in which the sides of the waveguide are buried.
A conventional buried stripe type semiconductor laser device having a buried hereto structure shown in FIG. 6. This buried stripe type semiconductor laser device can emit laser light oscillating stably in a single transverse mode at low threshold currents. The semiconductor laser device is fabricated in the following way.
An n-InP buffer layer 52, a GaInAsP active layer 53, a p-InP cladding layer 54, and a p-GaInAsP cap layer 55 are sequentially grown on an n-InP substrate 51 by a suitable crystal growth method (a first crystal growth process). Then, a stripe-shaped mesa portion (with a width of about 1 .mu.m to 2 .mu.m) is formed along the cavity length of the resonator by photolithography and chemical etching techniques. Subsequently, a p-InP first current stopping layer 56 and an n-InP second current stopping layer 57 are sequentially grown by a suitable crystal growth method in such a manner that the mesa portion is thereby buried (a second crystal growth process). Finally, an n-side electrode 58 is formed on the back of the n-InP substrate 51, and a p-side electrode 59 is formed on the top surfaces of the p-GaInAsP cap layer 55 and n-InP second current stopping layer 57, whereby the semiconductor laser device shown in FIG. 6 can be obtained.
In order to fabricate the buried stripe type semiconductor laser device, two step processes for crystal growth are required, as described above. This complicates the fabrication process sequences and makes it impracticable to form the required device structure in an easily reproducible way.
A buried stripe type semiconductor laser device which can be fabricated by a single-step crystal growth process is proposed (Japanese Laid-Open Patent Publication No. 64-25590). FIG. 7 shows a cross-sectional view of the buried stripe type semiconductor laser device. This semiconductor laser device is fabricated in the following way.
First, a stripe-shaped mesa portion is formed on an n-GaAs substrate 61 having a main surface with a (100) crystal orientation, in the &lt;011&gt; direction by using a suitable etching method. Then, an n-GaAs film 62', an n-AlGaAs film 63', a GaAs film 64', and a p-AlGaAs film 65' are sequentially grown on the n-GaAs substrate 61, by a suitable method for crystal growth (e.g., metal organic vapor phase deposition method). In this process step, on the stripe-shaped mesa portion is formed a multilayer film consisting of an n-GaAs buffer layer 62, an n-AlGaAs first cladding layer 63, a GaAs active layer 64, and a p-AIGaAs second cladding layer 65 and surrounded by (111)B facets. Then, in succession, an n-AlGaAs current stopping layer 66, a p-AlGaAs cladding layer 67, and a p-GaAs cap layer 68 are sequentially grown so as to bury the multilayer film. Finally, an n-side electrode 69 is formed on the back of the n-GaAs substrate 61, and a p-side electrode 70 is formed on the top surface of the p-GaAs cap layer 68, whereby the buried stripe type semiconductor laser device shown in FIG. 7 can be obtained.
In the buried stripe type semiconductor laser device shown in FIG. 7, the top surfaces of individual epitaxial layers grown sequentially over the semiconductor substrate rise along the stripe-shaped mesa portion. Such a tendency is more noticeable in the semiconductor laser devices using an InP substrate.
In fabricating the semiconductor laser device using the GaAs substrate, when epitaxial layers are grown on the GaAs substrate on which a stripe-shaped mesa portion has been formed, the rate of crystal growth on the (111)B facets is substantially zero and, therefore, continuation of crystal growth after a multilayer film surrounded by the (111)B facets will result in an epitaxial growth progressing in such a way that the difference in level between the GaAs substrate and the mesa portion is reduced. In contrast, where the InP substrate is used, the rate of crystal growth on the (111)B facets is considerably greater than in the case where the GaAs substrate is used and, therefore, epitaxial growth is continued while the level difference between the InP substrate and the mesa portion is held as it is. Thus, the resulting configuration will be as shown in FIG. 8.
The buried stripe type semiconductor laser device shown in FIG. 8 comprises an InP substrate 71 having a stripe-shaped ridge, a first multilayer film (including a cladding layer 72 and an active layer 73 formed on the cladding layer 72) formed on the top surface of the stripe-shaped ridge, a second multilayer film including layers 72' and 73' formed on the InP substrate 71, a cladding layer 74 for burying the multilayer films, a contact layer 75 formed on the cladding layer 74, an SiO.sub.2 insulating layer 76 formed on the contact layer 75, a p-side electrode 78 formed on the SiO.sub.2 insulating layer 76 and the contact layer 75, an n-side electrode 77 formed on the back of the InP substrate 71.
Since the semiconductor laser device shown in FIG. 8 is such that a portion above the mesa portion is in an uprising condition, the laser device cannot be mounted in position with the epitaxial layer side held down. As such, the semiconductor laser device is often subject to thermal influences due to increased resistance to heat and, in addition, the mesa portion is susceptible to damage upon mounting of the device, with the result that some strain is generated in the active region, which leads to decreased reliability.
In order to stabilize the transverse oscillation mode and reduce the threshold current, it is necessary to provide a structure which can prevent the spread of injected currents so as to allow the electric currents to concentratively flow in a laser beam emitting active layer and can efficiently confine the laser lights in the narrow region. In order to fabricate the buried stripe type semiconductor laser device in an easily reproducible manner and to obtain a high yield, the structure must be such that it can be constructed through a single crystal growth process. With semiconductor laser devices of such a structure, in order to define the region for current injection, there must be provided a stripe-shaped electrode or a stripe-shaped impurity diffused region.
FIG. 9 shows a buried stripe type semiconductor laser device having the stripe-shaped impurity diffusion region disposed therein. This semiconductor laser device is fabricated in the following manner.
A stripe-shaped ridge 131 is first formed on an n-InP substrate 130 having (100) surface in [011] direction. Then, an n-InP film 132', a GaInAsP film 133', and a p-InP film 134' are sequentially grown on the n-InP substrate 130 on which the stripe-shaped ridge 131 is formed. In this process step, on the upper surface of the stripe-shaped ridge 131 is formed a multilayer film of a double hereto structure having a triangular sectional configuration perpendicular to the cavity length of the resonator and which is surrounded by (111)B facets, that is, a multilayer film whose slat faces are (111)B facets grown from opposite ends of the stripe-shaped ridge 131. This multilayer film consists of an n-InP first cladding layer 132, a GaInAsP active layer 133, and a p-InP second cladding layer 134. In this case, the p-InP film 134' grows to a thickness such that the p-InP film 134' does not go above the GaInAsP active layer 133 of the double hereto structure formed on the upper surface of the stripe-shaped ridge 131. However, InP crystal grows also on the (111 )B facets of the multilayer film, therefore, as shown in FIG. 9, the p-InP film 134' covers the multilayer film of the double hereto structure.
Subsequently, an n-InP buried layer 135 and a p-GaInAsP contact layer 136 are sequentially grown so as to bury the stripe-shaped ridge 131 covered with the p-InP film 134'. Then, Zn is diffused in a stripe pattern so that it reaches the p-InP second cladding layer 134, in order to form a Zn diffused region 137 for current injection. Finally, an n-side electrode 138 is formed on the back of the n-InP substrate 130 and a p-side electrode 139 is formed on the surface of the p-GaInAsP contact layer 136. Thus, the semiconductor laser device shown in FIG. 9 is obtained.
In the semiconductor laser device shown in FIG. 9, the n-InP film 132', GaInAsP film 133', and p-InP film 134' are formed on a plateau at both sides of the stripe-shaped ridge 131 as stated above. Therefore, in order for injection current to concentratively flow into the multilayer film of the double hereto structure formed on the top surface of the stripe-shaped ridge 131, the stripe-shaped Zn diffused region 137 was formed to extend from the p-side electrode 139 to the p-InP second cladding layer 134.
However, the p-InP film 134' is grown also on the (111)B facet of the multilayer film, and this does not permit satisfactory current confinement by the p-n junction reverse bias generated in the burying layer. As a consequence, considerable current leaks through the p-InP film 134', which increases threshold currents.